Gordon at We Fly Spitfires has done an interesting piece on levelling. I started doing a wall of text as a response, and felt that the reply would be better suited as a proper entry.
I still haven't got the hang of trackbacks/backlinks between Blogger and Wordpress, so here is a random trackback link.
There was a poll embedded, and I answered that I didn't care how long it took me to level. But that would only be partially true.
When playing a new game, levelling gives a chance for the game creators to tell the story. If there are factions, you may do it a couple of times to get both sides of that story.
When playing a new class, levelling the way nature (or Blizzard or whoever) intended gave me a good grounding in the mechanics of that class. But once you've completed it once, you may not want to do it again for that class, unless you started on a PvE server, and would like a PvP experience. Solo levelling a warrior in a PvP environment was something I consider to be almost the ultimate in hardship, especially if you did it as Prot. I could only imagine doing the same as a holy priest to be second in difficulty.
But after a while, you get used to the various classes, and you just want to jump in with a prebuilt class. Features like multiple roles / dual spec melt an argument for levelling for the sake of class mechanics.
After all, here you are, at level 85. Perhaps you've levelled as Arms the whole way, and now you want to get into tanking. There's no smooth transition if you've never tanked before. You just take the lowest level dungeon you can, even though you're overpowered, and start applying the basics you've read about. Eventually, you'll hit the harder dungeons for your level and will actually be tanking properly (you hope) without being over powered. Then you end up with the gear scale, as each of the harder dungeons require better gear. There's a skill scale in there as well, but usually, you'll hit gear as a limiting factor as the norm. Skill scale is something for the extremes.. i.e. you're just starting, or you're running hard modes in the premier raid.
Let's throw a RIFT comparison in there, too. RIFT is a bit easier going. It's not just dual spec, it's up to 4 specs. For a cleric, you may have a defensive build, an offensive build, a healing build and a PvP build. Though due to the cost of the roles, you may only have 3 role slots to select from for quite some time. Further more, you may only be interested in the offensive build. At some point, you may pick up a healing role, and there will be no nice levelling experience to help you use it. You may even just decide to pick a different primary soul at level 50, and there will be no nice levelling experience to help you learn it. You just jump in the deep end, grind some mobs, kill some players until you get comfortable with it. These are examples where levelling to learn a class comes unstuck.
Back to the original question, however, of do I care how long it takes to level? So far, not if I'm experiencing the story for the first time. And not if I'm learning a class for the first time.
But now I'm in a tricky position. I'm about to hit level 40 in RIFT on my cleric. Most of the guildies are now 50 or in the high 40s. I'm super keen to get up there and enjoy content with the guildies instead of going through the motions of levelling. On the other hand, I'm about to enter a new zone, so there's a story being unravelled for that zone, and I do like to read the quest text. I'm sure that after I hit 50, there is still going to be levelling to do, and I'm still going to take the time to read the quest text. I only get a couple of hours play a night, so levelling is taking a bit longer for me that the rest. I'm just going to have to exercise my patience.
PvP is going to be interesting. Warfront PvP, that is. I've only done the one warfront, and I'll need to do a whole bunch of I want a PvP soul. There's a really good chance I won't start the PvP properly until I hit level 50. It's going to be painful, I know. Going on to the battlefield with on Valor (the equivalent of Resilience), I'm going to be made into prawn paste for quite a few weeks until I can gear up.